A large crowd of Julian Assange supporters gathered outside court as the WikiLeaks founder started his fight against extradition to the US, where he is accused of helping to leak hundreds of thousands of classified documents.

Demonstrators, including Assange's father John Shipton, arrived at Woolwich Crown Court on Monday, with some holding placards calling for his immediate release.

Some said they had arrived as early as 6am, with signs reading: "Don't shoot the messenger, free Assange" and "Free Julian Assange, Nobel Peace Prize nominee". Tents were erected on nearby grass.

Mr Shipton claimed on Sunday that his son had been "harassed" by a prison cell search on the eve of the first day of his full extradition hearing before District Judge Vanessa Baraitser.

Extradition case: Wikileaks founder Julian Assange.
Picture:
PA

Assange, 48, is wanted in America on 18 charges related to the publication of US cables a decade ago. If found guilty could face a 175-year prison sentence.

The Australian is accused of working with former US army intelligence analyst Chelsea Manning to leak classified documents.

In the lead-up to the hearing, Assange, who is being held in Belmarsh Prison in south-east London, has received high-profile support including from Pink Floyd's Roger Waters, Greek economist Yanis Varoufakis and fashion designer Vivienne Westwood.

He has also been supported at previous hearings by Rapper MIA and filmmaker John Pilger.

After a visit to the prison on Sunday, Mr Shipton criticised the "plague of malice" which he said "emanates from the Crown Prosecution Service" towards Assange.

He urged that his son be allowed bail, telling reporters: "For the life of me I can't understand why Julian Assange is in jail having committed no crime, with family here that he can come and live with."

Mr Varoufakis said Assange was in a "very dark place" due to spending more than 20 hours a day in solitary confinement and called for the extradition to be stopped "in the interests of 300 years of modernity, 300 years of trying to establish human rights and civil liberties in the west and around the world".

More than 40 international legal experts have written to Prime Minister Boris Johnson demanding the "rule of law be upheld", claiming he has not had proper access to his legal team.

The letter was handed in to 10 Downing Street on Saturday and also urged the British legal community to act "urgently" to secure Assange's release.

Assange has been held on remand in Belmarsh prison since last September after serving a 50-week jail sentence for breaching his bail conditions while he was claiming refuge in the Ecuadorian embassy in London.